Water treatment systems such as water softeners and deionizers are used in both industrial and residential applications. In the case of residential applications, water treatment units are typically small in size and include one or two resin tanks which treat the incoming water. When a single resin tank is used, water service to the household is usually interrupted when regeneration is needed. In a two tank treatment system continuous water service is provided by having one tank on-line at all times. The other tank is regenerated and maintained off-line until the first tank is exhausted and requires regeneration.
Water treatment units used in industrial applications usually operate on the same principal as residential treatment units. However, for at least some industrial applications, the quantity of treated water needed mandates either a much larger resin tank or a multiplicity of tanks in order to support the required flow rate.
When a single resin tank is used in an industrial application, it normally must be very large in order to provide sufficient service time before requiring regeneration. When regeneration is required, the flow of regeneration chemistry and rinse water out of the tank can be substantial and may require the need for even larger equipment to treat the regeneration effluent. In a single vessel system, the required treatment equipment for the regeneration effluent can be very costly and remains idle most of the time.
When multiple tanks are employed, the flows of regeneration waste are smaller than those for a single tank system and therefore the equipment needed to deal with the regeneration chemistry discharged during a regeneration cycle can be smaller. However, with this type of treatment system, it can be difficult to maintain water quality since with prior art systems, it is difficult to insure that a given tank is taken off-line and regenerated before reaching exhaustion. At least with some prior art systems, it is possible for a second tank to reach exhaustion while a first tank is still in a regeneration cycle. If this should happen, water quality will suffer substantially if the second exhausted tank is left on-line. Alternately, if the second tank is taken off line, the flow rate of treated water will be substantially reduced (if the system includes three or more tanks) or will be terminated (if the system includes only two tanks).
One suggested way of avoiding this type of problem is to regenerate the tanks in sequence and at an interval that will insure that each tank is regenerated well before it is exhausted. With this type of control, regeneration normally occurs more frequently than needed and as a result, excessive regeneration chemistry may be consumed which substantially raises the cost of operating the system. The increase cost is not only due to the cost of the excess chemistry used, but also the cost associated with the treating of the regeneration waste discharged during the regeneration cycle. As indicated above, it is quite common to use supplementary treatment equipment to treat the regeneration discharge. As the quantity of regeneration chemistry increases, the cost associated with treating this regeneration effluent also increases.